There’s no ‘standard’ definition of what a C2 is. There are 2 competing systems for octave-numbering, and they differ by one octave, so it could be that Yarns and your MIDI keyboard use different numbering systems.

To be clear, this is coming from a Beatstep Pro. The measure voltage of the Beatstep Pro analogue pitch out (cv 1 v/oct) is 3 vdc (c3) The measured volts of the Beatstep Pro midi to Yarns to pitch out ( cv 1 v/oct) is 2 vdc (c2). I have a very good Fluke meter and is accurate.